The present invention relates to an underground drainage facility for gathering flowing water such as, for example, rain water and the like, which has flowed from water channels including creeks, into a main underground inlet or input conduit, with the flow then being introduced into a drain pump installation so as to enably discharge to a determined drain portion such as a river or the like, and, more particularly, to an underground drainage facility for a wide area drainage facility in a city and suburbs, and with the underground drainage facility including reservoirs for storing inflowing drainage water.
Since the amount of paved surface area in cities and suburbs has considerably increased, the proportional amount of rain water which would have, in the past, penetrated into the surface of the earth decreases and, consequently, the quality of flowing water in the underground drainage facility increases in a very short time period. In order to cope with the increase in the quantity of flowing drainage water, a variety of countermeasures have been taken such as, for example, a diameter of the main inlet or input conduit of the underground drainage facility has been increased with a number and/or capacity of drain pumps have also been increased. Furthermore, there has been a tendency toward a very deep underground drainage installation, with the main inlet or input conduit being laid deep in the earth in order to sufficiently increase the hydraulic radiant. By the term "very deep" as used herein is meant an underground space surrounded by a 30 to 50 m or deeper hard ground free of any influences of upper structures.
In very deep underground drainage facilities having the main input or inlet conduit laid deep underground, it is feasible to cause the main inflow conduit to have a water storage effect by forming reservoirs at the same level or depth or at a level deeper than that of the main inlet or input conduit or by enlarging the cross-sectional area of the main input conduit so as to cope with a temporary increase in the quantity of the inflowing water.
However, the very deep underground drainage facility having reservoirs at a level deeper than that of the main input conduit, and with the main input conduit being capable of enabling a water storage, suffers from the disadvantage that the input or inlet vertical shaft establishing a connection between the water channels formed on the surface of the earth and the main inlet or input conduit has only a small storage capacity. Therefore, the operation of the drain pump must begin before the downstream side of the main inlet or input conduit is filled with water. Accordingly, the required water pressure head for the pump must be must be raised in accordance with the depth of the buried main inlet or input conduit, causing the energy cost and cost of installation to be increased as well as increasing the size of the facility thereby resulting in an uneconomical installation.
The increasing of the diameter of the main input or inlet conduit and the forming of the water reservoir deep underground increases the amount of necessary excavation work to be performed at a very deep location. Therefore, problems arise in labor effectiveness, the length of the required construction time, and excessively high construction costs.